How to force the temperature at the interface equal to the saturation temperature
 

I am working on the two-phase flow in a pipe. Primary phase is water vapor and the secondary phase is water liquid. I am trying to force the temperature at the interface equal to saturation temperature using large source term method. Large source term method is to set Sp equal to a very large number so that ap will be very large in energy equation. What i found in Fluent is the ds[eqn] doesn't represent Sp , when I assign a big value for ds[eqn], Fluent didn't response.
Could anyone help me with it? Where should i change the Sp?